The present invention relates to a pig iron refining process in which industrially pure oxygen is blown in by means of at least one tuyere located below the surface of the molten metal in a converter.
In known installations the tuyere is generally in the bottom of the converter and comprises two co-axial ducts, the inner duct being arranged to convey oxygen whereas the space between the inner duct and the outer duct is designed to convey a fluid, preferably an endothermic-decomposition fluid such as a hydrocarbon liquid or gas, to protect the tuyere and the refractory materials against the action of the oxygen.
Input and output parameters are monitored and input parameters are controlled in order to obtain steel of a desired quality. Fixed input parameters include the temperature and composition of the pig iron; variable input parameters include oxygen flow rate and the amount and timing of additions to the molten metal; output parameters include the temperature and composition of the waste gases. The desired quality may be, for example, the content of at least one component of the steel.
A number of methods of controlling the refining operation when blowing oxygen through the bottom of the converter have already been suggested, for example methods based on material and heat balances and a mathematical model for calculating charges.
Theoretically, such methods permit the desired composition and temperature for the refined metal to be regularly obtained upon turn-down of the converter. However, a certain scatter of the results thus obtained occurs in practice. The dispersion may be due to a lack of information on the charged materials, for example on the weight or the precise composition of the pig iron, the scrap, or the lime. To remedy such a situation, premature turn-down of the converter is generally provided, the iron content and the temperature of the slag are rapidly measured, and blowing is re-started with or without additions and is continued for the time necessary to obtain the desired composition and temperature of the steel.
The results thus obtained have been found to be satisfactory. Scatter, which generally occurs when no premature turn-down is practiced, is substantially reduced. However, premature turn-down of the converter has the disadvantage of prolonging the duration of the refining operation.